Field of the Invention
Embodiments of the present invention generally relate to a micro-electromechanical system (MEMS) and a method for manufacture thereof.
Description of the Related Art
MEMS digital variable capacitor (DVC) devices are based on a moveable MEMS plate with a control-electrode above (i.e., pull-up electrode, pull-off electrode or PU-electrode) and below (i.e., a pull-in electrode, pull-down electrode or PD-electrode) the moveable MEMS plate, as shown schematically in FIG. 1. These electrodes are covered with a top and bottom dielectric layer. In addition there is an RF-electrode below the moveable MEMS element. Between the moveable plate and the RF-electrode there is a gap that is modulated by the voltages applied to either the PU or the PD-electrode with respect to the plate-electrode. These voltages result in electrostatic forces, which pull the MEMS element either up or down in contact to provide a stable minimum or maximum capacitance to the RF-electrode. In this way the capacitance from the moveable plate to the RF-electrode can be varied from a high capacitance state Cmax when pulled to the bottom (See FIG. 2) to a low capacitance state Cmin when pulled to the top (See FIG. 3).
FIG. 4 shows a more detailed cross-section view of a MEMS DVC device. The moveable plate consists of 2 layers (i.e., the bottom-plate and the top-plate) which are connected to each other via a plurality of posts. This combination of plates and posts creates a semi-rigid plate which is hard to bend. The plate is anchored to the substrate via a flexible leg-structure which allows relatively low operating voltages to operate the DVC device in the Cmin or the Cmax state.
FIG. 5 shows the CMOS waveform controller which generates the required voltages Vpu and Vpd to be applied to the MEMS DVC device on the PU and PD-electrode. The plate potential needs to be referenced to the CMOS ground potential so that the applied voltages on the PU and PD electrodes generate the required electrostatic forces to move the plate up or down to the Cmin or the Cmax position. In applications where the plate electrode needs to be RF-floating this reference is made using a high-value resistor Rplate between the plate-electrode and CMOS ground (See FIG. 5).
FIG. 6 shows the CMOS waveform controller where the plate potential is referenced to the CMOS ground potential using diodes Dplate. In this application, the plate-electrode is typically on RFGND. A combination of Rplate and Dplate can also be used.
These electrical connections between the moveable plate and CMOS ground are required for the electrostatic actuation. However, these connections also can create issues during processing of the 2-layer plate. Specifically, having the moveable plate connected to CMOS ground during processing can cause stresses in the moveable plate which can shift the actuation voltages Vpu and Vpd out of spec, significantly impacting the wafer yield.
Therefore, there is a need in the art to provide a means to avoid this issue and obtain more controlled actuation voltages.